(1) Field of the Invention
The present invention relates to a casting nozzle and, more particularly, to a nozzle which is mounted at the lower part of a ladle or tundish so as to guide molten steel into a mold for continuously casting steel.
(2) Description of the Prior Art
A casting nozzle used for casting molten steel is used in severe conditions of high temperatures. Various studies have been made on refractories for the casting nozzle. A tundish nozzle for guiding molten steel from a tundish to a mold for continuous casting of steel is considered to be one of the most important devices used in continuous casting. The casting nozzle which communicates the tundish with the mold must have good heat insulating characteristics and must be able to prevent oxidation of molten steel, turbulent flow of molten steel, or the mixing in of a slag. The nozzle must also be durable, and so, economical.
Alumina-graphite based, zirconia-based or zircon-based casting nozzles are conventionally known. However, each one of these nozzles has both advantages and disadvantages and is not wholly satisfactory.
An alumina-graphite based-casting nozzle contains clay as a binder and is thus less resistant to thermal shock, relatively fragile, and inferior in corrosion resistance. Especially in the case of an immerging nozzle, significant corrosion takes place upon contact with molten steel, which results in erosion or enlargement of the nozzle diameter. Although a zirconia- or zircon-based casting nozzle is resistant to erosion, it is subject to deposition thereon of metals especially aluminum and oxides of aluminum in the molten steel. This causes reduction in the nozzle diameter during casting and may finally result in closure of the nozzle.
An immerged nozzle of fused quartz has also been recently developed. If such an immerged nozzle of fused quartz is used to cast molten steel which contains 1.2 to 1.5% manganese, it is subject to significant melting loss.
In view of this, the present inventors previously proposed a continuous casting nozzle which is prepared by sufficiently kneading with a binder 42 to 93% by weight of alumina power, 4 to 44% graphite powder, and 2 to 23% silica powder, and pressing the resultant mixture with a rubber press. A patent application for claiming the right on this continuous casting nozzle was published on Dec. 12, 1972 and was granted a patent on May 13, 1979 as Japanese Pat. No. 955,778.
This continuous casting nozzle has corrosion resistance, erosion resistance, clogging resistance and spalling resistance which are superior to those of the conventional casting nozzles. However, the patent as mentioned above teaches the use of a silica powder such as amorphous silica or quartz powder as a raw material. Although this patent also suggests the use of fused silica, 5% by weight of fused silica is used. This patent does not limit the sintering temperature of the pressed casting nozzle. However, according to further studies made by the present invention, if a fused silica powder which has a small coefficient of thermal expansion and a small coefficient of thermal conductivity is used, the resultant casting nozzle is superior in heat resistance and spalling resistance to casting nozzles using other silicas. Good results are obtained if at least 10% of such a fused silica is used and if the sintering temperature of the pressed casting nozzle is between 800.degree. and 1,200.degree. C.